Weird shadow colors / saturation in a Copper material (Principled BSDF)
Filter Glossy
After doing a little bit of testing, I found that the problem seems to stem from this little booger in the render settings.

Filter Glossy is designed to reduce noise in reflective caustics by &quot;adaptively blurring glossy shaders,&quot; but it seems that one of the side effects is very inaccurate recursive reflections.
Tests:

All 32 samples and denoised with OpenImageDenoise.
12 glossy bounces
Glossy BSDF set to Multiscatter GGX
Color: RGB (1.0, 0.485, 0.352)
Roughness: 0.15

Filter Glossy set to 10.0 (maximum):

Filter Glossy set to 1.0:

Filter Glossy set to 0.1:

Filter Glossy set to 0.0: